Project Summary Many lower limb amputees wear prosthetic feet that store and release energy through articulation of an elastic keel. The function of these energy storage and return (ESAR) feet are to provide mechanical energy for propulsion that was once provided by the muscles crossing the ankle joint. Clinicians are well aware that foot stiffness strongly influences the mechanics of amputee gait and prescribe ESAR feet with manufacturer-identified stiffness levels based on body weight and self-reported activity level. However, very little objective biomechanical data exists to guide the prescription of one stiffness level versus another and amputees continue to experience significant biomechanical and metabolic gait deficits relative to non-amputees and often develop secondary musculoskeletal disorders in both the intact and residual limbs due to abnormal limb loading. We believe many of these adverse results can be mitigated if the relationships between ESAR properties and the biomechanical and metabolic response of amputees are understood. However, to date no study has systematically varied prosthetic foot stiffness across a wide range of values to identify these relationships. Another contributing factor is that to date the majority of research evaluating the effectiveness of ESAR feet has been performed during straight line walking. However, non-sagittal plane activities such as turning are prevalent in daily living and create a different set of design requirements for ESAR feet that has yet to be evaluated. To address this important clinical problem, the overall goal of this research is to perform a systematic study of the influence of prosthetic foot-ankle stiffness on amputee gait performance during straight-line walking and turning to define this relationship and explore its use as a predictive prescription tool.. A unique element of the proposed work is the use of a rapid prototyping system to quickly fabricate patient- specific prosthetic feet with a wide range of stiffness properties. Specific Aim 1 will identify the effects of sagittal plane foot stiffness on amputee gait through a human subject experiment with unilateral transtibial amputees (n=20) wearing prosthetic feet with five different stiffness levels during straight-line walking. We will use rapid prototyping techniques to fabricate feet that match the stiffness properties in both the sagittal and coronal planes of each subject's own clinically prescribed prosthesis and four others (125% and 150% more or less stiff). The patient-specific properties of each foot will be precisely determined using a robotic gait simulator. Blinded to stiffness conditions, each subject will walk in a straight line on the prostheses to test specific hypotheses relating sagittal plane stiffness to expected changes in gait mechanics and metabolic cost. Specific Aim 2 will identify the effects of coronal plane foot stiffness on amputee gait using a similar experimental design and methods. Unilateral transtibial amputees (n=20) will wear prosthetic feet with five different coronal plane stiffness levels (as-prescribed and 125% and 150% more or less stiff) during straight-line walking and while walking along a curved path. Data from this second experiment will enable hypothesis testing relating coronal plane stiffness to expected changes in gait mechanics. We hypothesize that amputee mobility can be improved by optimizing the stiffness of their prosthetic feet in both the sagittal and coronal planes. This study will provide a thorough understanding of how prosthetic foot properties effect amputee gait and help us achieve our long-term goal of developing predictive prescription tools for the clinic that will result in greater amputee mobility, independence, and quality of life.